THE LOSSES OF CALCIUM IN COOKING KALE
by
MARY ELIZABETH BERTRAM
A THESIS
submitted to the
OREGON STATE AGRICULTURAL COLLEGE
in partial fulfillment of
the requirements for the
degree of
MASTER OF SCIENCE
June 1936
^
APPROVED:
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P/toftessoF of Foods and Eutrition
In charge of Major
1,
Associate Pirofessor of Foods and nutrition
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Chairman of School Graduate Committee
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Chairman of College Graduate Council.
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ACKNOWLEDGMENT
I wish to express my grateful appreciation to
Professor Jessamine C. Williams, Head of the Department of
Foods and Nutrition and to Dr. Margaret L. Fincke,
Associate Professor of Foods and Nutrition, and to others
who have given helpful suggestions and encouragement during
the period of this study.
TABLE OF CONTENTS
Introduction
Factors Influencing Utilization of Calcium
1
3
Calcium Absorption
6
Losses of Minerals in Cooking
9
Methods of Cooking Kale
Experimental
Plan of Experiment
-.ll
„... ,.13
.13
Preparation of Materials
-.13
Method of Analysis
...14
Preparation of Reagents.
-.15
Results and Discussion
Summary and Conclusions
Bibliography
..16
.20
...,25
THE LOSSES OF CALCIUM IN COOKING KALE
INTRODUCTION
Sherman has stated that the average American diet is
more likely to be deficient in Calcium than in any other
mineral.
vidual.
Calcium is one of the requirements of every indiIt is needed as a constituent of the skeletal
tissues, the bones and teeth, and also is an essential factor in the construction of the soft tissues, blood cells
and nerve cells.
The growing child and the pregnant woman:
need a much larger supply in their diet than the normal adult, to allow for growth;., while lactation requires an even
larger amount,,
(21) (43).
The body's supply of calcium comes through its food,
although a very small amount may be found in some water,
A
liberal supply of milk is the surest means of providing an
adequate intake of calcium, especially for children.
As a
result of balance experiments a dietary standard has been
set as a quart of milk per child per day and a pint of milk
per day for each adult.
These standards are the results of
the studies of Sherman and Hawley (40) who found that storage of calcium in children was raised 70% by increasing the
intake of milk from 750 to 1000 grams a day.
The results
2
of a study by Daniels (15) indicate that one pint of milk
will supply sufficient calcium for the well nourished child
between 3 and 5 years, provided the diet is rich in protein,
phosphorus and vitamins.
Stearns (42) states that as a re-
sult of a series of studies upon the retention of calcium by
children fed different amounts of milk, the total daily retention of calcium by infants was as great or greater than
the retention of the older children for any given quantity
of milk studied, but in all cases a higher retention of calcium resulted when a quart of milk was fed than with a pint.
Many studies, including those of Lininger, (27) and
of Leighton and Clark (26), have shown improved growth and
physical and mental well being in school children, when
additional milk was added as a supplement to their usual
diet..
The milk was given to the children as a mid-morning
lunch at school.
Large groups were used and no record was
kept of the home diet.
FACTORS INFLUENCING THE UTILIZATION OF CALCIUM;
3
Other foods vary a great deal in the amount of calcium found in them;, also, the calcium content of different
samples of one food may vary considerably; (11) likewise,
the availability of the calcium of. different foods may not
always be the same.
Many balance experiments have tested the availability
and efficiency of vegetables as sources of calcium in the
diet.
Rose, (38) in a study on four young women eating a
diet in which 35$ of the calcium was furnished by carrots,
found a positive calcium balance in three cases.
In the
fourth case, the loss of calcium was small and was accompanied by a digestive upset.
Blatherwick and Long (4)
found that the calcium from mixed vegetables, including
cabbage, squash, celery, asparagus, etc., was well utilized,
and their data indicate that the calcium derived from vegetables is capable of meeting the maintenance needs of man.
Rose and MacLeod (37) in an experiment with adult women on
a diet in which 73$ of the calcium was derived from almonds,
found that calcium equilibrium could be secured about as
efficiently as when milk or carrots furnished the same amount.
When- 85% of calcium was furnished by almonds the
utlization was not quite as good.
McLaughlin (29) found
that on a diet in which 70$ of the calcium came from
spinach, six out of seven adult women maintained positive
calcium balances, while calcium equilibrium was maintained
4
in the seventh.
However, although the calcium content of
the diet containing spinach was greater than that of the
milk diet with which it was compared, the retentions in all
cases were lower on the spinach diet.
H'allon, Johnson, and
Darby (31) concluded that the calcium of lettuce, which
furnished 93$ of the total calcium of the diet, was perhaps
even better utilized than that of pasteurized whole milk.
Fincke and Sherman. (16) working with rats found that when
half the calcium of the diet was supplied by kale, the calcium was nearly as well utilized as when it all came from
milk.
When spinach was substituted for kale to provide the
same amount of calcium, the calcium of spinach was utilized
poorly, evidently due at least in part, to the presence of
oxalic acid.
Kramer, Potter, and Gillam (25) in a study on ten
adult women found that calcium from: ice cream was at least
as well utilized as that from milk.
Kramer, Latzke, and
Shaw (.24) reported that fresh and evaporated milk and quickly boiled milk are superior to pasteurized or dried milk as
sources, of calcium for both children and adults.
A very good source of calcium is found in Aires rican
Cheddar cheese.
Mallon, Johnson, and Darby (30) in a meta-
bolism study on two women determined that the calcium of
cheese was as well utilized as the calcium from pasteurized
whole milk.
Very few fruits may be ranked as good sources of
5
calcium.
Chaney and Blunt (10), however, found that cal-
cium assimilation was decidedly benefited when orange juice
was added to the diet of two children..
This may have been
due to some other factor than an increase in calcium.
Soy beans which are used to a great extent in China
were shown by Adolph and Chen (1) to be equally as effective a source of calcium as cow's milk..
Pittman (35) stud-
ied the utilization of the calcium of the navy bean.
When
80 to 85% of the calcium of the diet was supplied by beans,
negative balances were obtained in all cases.
She suggests
that this may be due to association of calcium with indigestible material.
However, the total amount of calcium in
the diet was low.
Burton (6) reported that the calcium of oats was not
as well utilized as that of wheat.
The oat diet contained
a smaller amount of calcium than the wheat diet, but
Burton states that the decrease in retention of calcium
on the oat diet was even greater than the decrease in intake of calcium between the two diets.
true for both adults and children.
She found this
CALCIUM ABSORPTION.
6
Many factors have been found to affe-ct calcium absorption..
Since calcium compounds are soluble in acid, any-
thing which would lower the pH of the digestive tract would
increase absorption of calcium.
Roe and Kahn (36) state
that the serum calcium of human subjects is definitely elevated by administration of calcium lactate in aqueous solution.
Kline and Keenair (23), feeding lactose to chicks on
a rachitic ration, found it had a favorable effect upon
calcium absorption; and Gross (17), feeding lactose to dogs,
believed lactose produced better absorption of calcium from
the intestinal tract.
Both lactic acid and lactose would
increase the acidity of the intestinal tract.
An alkaline
condition in the digestive tract, on the other hand, tends
to decrease the absorption..
As the pH of material leaving
the stomach approaches neutrality, the phosphate and carbonate ions tend to combine with calcium.
When the pH is
above 7, the calcium is precipitated in the form of tertiary
phosphate and also carbonate; also, the fatty acids react
with calcium to form insoluble soaps.
These products are
quite insoluble and very poorly absorbed, according to
Cantarow (8).
Coons, Coons and Schiefelbusch state that the bestJ
retention of calcium in human pregnancy is found when the
diet is base forming (13).
Morgan and others, (33) found
the greatest storage of calcium occurred on neutral or
base forming diets.
Their work was with dogs.
Thus a diet
7
which is acidic in the digestive tract and neutral or basic
in the tissues seems to favor calcium retention..
There are different opinions in respect to the effect
of fiber upon the absorption of calcium.
Bloom (5) in an
experiment with rats reported that fiber had no effect upon
the retention of calcium even when the total amount of fiber
was increased many times.
Ascham (2) states that increased
fiber in the diet seems to increase the calcium in the
feces of dogs, but her results were not conclusive..
The
good utilization of calcium in diets in which the calciumcame largely from carrots, almonds and other vegetables
would seem to indicate that the presence of fiber does not
hinder assimilation.,
Mallon, Jordan and Johnson state that fat cannot be
said to have a definite influence on calcium retention (32),
while Holt and Fales, (20) found that calcium absorption was
markedly reduced on a low fat diet in seven cases and gave
a negative balance in three.
Vitamin D in the diet seems to increase the absorption of calcium from the digestive tract, and abnormal conditions of high or low calcium and phosphorus in the serum
resulting in rickets, tetany etc., are alleviated and the
blood calcium tends to return to normal when vitamin D is
given (8).
However, Hart and Steenbock (18), state that
feeding half a pound daily of cod liver oil showed no favorable influence, upon the calcium assimilation of cows pro-
8
ducing large quantities of milk..
Bethker Kick and Wilder (3) believe that the ratio of
calcium to phosphorus in the diet is of importance, but the
work of Shohl (41) indicates that not only the ratio but
also the levels of calcium and phorsphorus are important.
Rickets may be produced in rats in which the ratio might be
within the normal range but the actual amounts of calcium
and phosphorus low.
Cantarow (8) states that when the calcium' and phosphorus in the diet is in good proportion, the presence of
other minerals does not seem to exert an unfavorable influence, which was confirmed in the case of magnesium- by
the work of Carswell and Winter (9),.
The secretions of the parathyroid gland also affect
calcium metabolism, but the way in which this is accomplished is still in doubt.
Increased secretion of parathormone
results in increased elimination of calcium and phosphorus
from the body; while decreased activity of the gland causes
the blood calcium to fall until a state of tetany results.
The exact mechanism of how this is accomplished, however, is
not certain: (8).
LOSSES OF MINERALS IN COOKING
9
Courtney,. Fales and Bartlett (14) found that a large
proportion of the mineral content of most vegetables is lost
in the- water used in boiling.
This loss varies a great deal
in different vegetables from a very small amount of calcium
in spinach to a loss of 28% of the calcium of carrots.
This
loss seemed to be decreased only slightly by reducing the
time of cooking.
Calcium apparently was the least affected
of all minerals.
A very great saving in mineral content is
effected by using the method of steaming.
Peterson and
Hoppert (34) state that the calcium loss in cabbage and
spinach was 10$ when they are steamed or pressure cooked and
20 to 30$ when they are boiled.
in iron and the least in calcium.
The greatest loss occurred
Certain vegetables showed
notably large losses of calcium, particularly cabbage.
Although steaming does conserve the minerals, King
(22) does not recommend cooking leafy green vegetables in
such a way because of the unpalatable product obtained.
The
smaller loss of minerals in cooking would be more than
counterbalanced by their loss in palatability.
Since the calcium of kale had been shown to be nearly
as well utilized as that of milk, this investigation was
undertaken to determine how much calcium was lost in cooking
kale by two common methods:
boiling in a large amount and
in a small amount of water.
Preliminary experiments were
first carried out to determine the ways of cooking kale to
icr
insure a palatable product.
METHODS OF COOKING KALE
II
Halliday and Hoble (19) and also Lowe (28) recommend
that the following different standards should be met in
green vegetable cookery:
it is desirable to retain the
minerals and vitamins, and also have a product pleasing in
texture, color and flavor.
The green coloring of the leaves is due to the presence of chlorophyll which is decomposed by heat and this
decomposition gives the leaf an olive green color.
The ex-
tent of the decomposition depends upon the time of cooking;
therefore, we want as short a cooking time as possible.
The
effect of heat upon chlorophyll is modified if the reaction
is alkaline.
Green vegetables are cooked with the cover off because part of the volatile acids will pass off with the
steam and the development of the olive green color is less
intense.
The highest percent of these acids pass off during
the first few minutes of cooking.
Vegetables which start to cook in boiling water show
less loss of minerals than those started in cold water.
Therefore a strong flavored vegetable is usually cooked in
a large amount of boiling water with the cover off, for as
short a time as possible.
In these preliminary experiments several methods of
cooking kale were tried and the products checked on a score
card for color, flavor and tenderness.
At first 100 gram
12
samples were used, but as this seemed too large for an average serving, the amount was later decreased to 57 grams
(2 oz.) per sample..
This was the amount used in the final
analysis of calcium.
Kale was prepared by steaming, by cooking in a casserole, by boiling in small and large amounts of water for
different lengths of time.
Each product was judged by
several persons and as a result of this scoring two methods
were selected..
In one, the 57 gram sample of kale was
cooked for 15 minutes in one cup of water, while in the
other, the same amount of kale was cooked for twenty minutes in two cups of water.
Boiling is perhaps the most
commonly used method of cooking and both these chosen methods gave palatable products.
The flavor was so much the
same that they could not be distinguished.
Therefore,
these two methods were selected for the experimental work.
EXPERIMEMTAL
Plan of experiment.
15
Since kale has been found to
contain a good supply of calcium in a form which is easily
assimilated by the human body, it was thought interesting
to determine the amount of calcium which is lost during the
cooking process.
Weighed samples from one lot of kale were
analyzed for the calcium content while raw and after cooking by the two methods outlined above..
Preparation of materials.
The variety of kale used
for analysis was Brassica oleracea, var.. acephal..
It was
grown on one of the Oregon State College farms and was furnished through the courtesy of the Horticultural Department,
being cut as needed throughout the months of October and
November. The kale was gathered and brought to the laboratory the day before cooking and weighed.
The leaves were
stripped from the petiole and tougher part of the midrib,
and the edible portion weighed.
About 50% of the kale as
obtained was found to be edible portion*
then thoroughly washed.
The leaves were
The kale was unusually clean so
that washing three times was always sufficient.
It was
spread on the table to dry over night, at the end of which
time it was free from visible moisture but not withered.
It was then weighed into 57 gram (2oz.) samples.
All cooking was done in small aluminum kettles.
The
kale was placed in these kettles and a measured amount of
boiling water poured over it.
Tap water was used througho:ut
14
for cooking purposes, as this would be comparable to conditions in the home.
The kale was cooked uncovered for the
required length of time, when it was drained in a wire strainer for thirty seconds, the cooking water, if any, being saved for analysis..
From each lot of kale, raw samples were
taken for analysis, and samples for cooking in each of the
two ways.
When the kale was cooked in one cup of water, the
liquid had all evaporated by the end of the fifteen-minute
cooking period; in the case of the samples cooked for twenty
minutes in two cups of water there was. about a half a cup of
liquid left in the cooking kettle.
The samples were dried in evaporating dishes in an
electric oven for six or more hours at 100° C.
then covered and stored until analyzed.
They were
The cooking liquid
was evaporated slowly over a flame and finally dried in an
electric oven.
The samples were ashed in an electric muffle furnace
until white.
red heat.
This ashing took four or more hours at a dull
The ash was dissolved in 5 cc. of concentrated
hydrochloric acid, filtered, and the solution made up to
250 cc. in a volumetric flask.
Method of analysis..
Aliquots of 100 cc. were with-
drawn from the solution by pipette and placed in 250 cc.
beakers; a small piece of litmus paper was dropped in and
ammonium hydroxide added drop by drop until the solution
became alkaline.
One drop of methyl red was then added and
15
acetic acid added drop-wise until the solution became pink.
The beaker was covered and heated to 85 to 95
C. when 25
cc. of saturated ammonium oxalate solution was poured in
slowly with constant stirring.
The beaker was then covered
and allowed to stand over night.
The solution was filtered and the beaker and coverglass washed with a 0.5 percent solution of ammonium oxalate,
then three times with distilled water, filling the filter
three-fourths full each time.
A hole was made in the filter
paper and the precipitate washed into a beaker with 100 cc.
of distilled water, and 10 cc. of 9 M sulfuric acid added.
The solution was heated to 75°to 80° C. and titrated while
hot with standard potassium permanganate solution.
permanent pink color was taken as the end point.
The first
The
reaction involved is:
2KMhQ4.+ 5CaC.204 ■* 8H2S04 -*
2MnS04 + K2S04 + 5CaS04 + SHgQ + SCOg
Preparation of Reagents.
Reagents were prepared when-
ever possible in sufficient amounts to last throughout the
entire period.
Two: potassium permanganate solutions were
made up which were approximately 0.02 M.
In the first, 1 cc.
oxidized an amount of calcium oxalate equivalent to- 0.00231
gm. calcium; 1 cc. of the second solution oxidized oxalate
equivalent to 0.00.281 gm. calcium.
The solution was stand-
ardized with C. P. Baker's Special Sodium Oxalate for standardization.
RESULTS MD DISCUSSION
16
The calcium content of the raw kale varied from a
weight of 0.1607 gm. to 0e.2295 gnu, per sample and averaged
0o.l826 gm.
As shcnn in Table I, this corresponds to a
centage of calcium in the sample of 0.,28$ to 0..40%c
average for the raw kale was 0..31%..
per-
The
Samples from the same
lot of kale checked closely..
Analysis of samples from one lot of Kale A, when
cooked 15 minutes in one cup of water, gave a weight of calcium varying from 0.1347 gm.. to 0.1512 gm.., with an average
of 0.1398 gm..
As shown in Table II, the percentage of cal-
cium in this lot varied from 0o.22$ to 0..26$ averaging 0.24$.
This represented the percentage calcium loss during the
cooking period of from 15 to 24$.,
Kale from Lot D, with the
highest content of calcium in the raw sample, showed a small
percentage loss in cooking,, 5% and 10$ loss as compared with
23$ and 24$ losses in the other samples.
When samples from Lot A were cooked twenty minutes in
two cups of water the analysis showed 0.0830 gm. and 0.1087
gm. of calcium or 0.14$ to 0.19%, or a loss in cooking
39$ and 54$ (Table III).
of
This is double the loss incurred
with kale cooked in a small quantity of water.
Samples
from Lot D cooked twenty minutes showed an analysis of
0 0.1385 gm. and 0.1390 gm, of calcium, a percentage calcium
content of 0..24$ and a 39$ loss in cooking.
Sample G gave
a weight of 0..1470 gm., of calcium or 0.26$, and showed a
17
8% loss in cooking.
The average of the different lots cook-
ed in this way was 0.1232 gm.. or 0.21% calcium and the
average loss was 36%.
The cooking liquid showed an average
weight of calcium of 0..0642 gnu or a percentage of 0.11%.
Variations in this analysis is due probably to one of two
things; the cooking utensils were not rinsed out and some
of the calcium would be precipitated upon the sides of the
kettle,, also, tap water was used in cooking the samples of
kale and this would have a varying content of calcium.
Table IV, giving the summary of results of the analyses on all samples, shows that Lot D gave the highest calcium content in the raw state and showed only a small loss
in the short period of cooking, 10% and 5%; but a 39% loss
in the longer cooking period.
Lot A with a content of cal-
cium of 0.31% in the raw vegetable, showed after 15 minutes
of cooking in one cup of water an average of 0..25% calcium
or a 21% loss.
After cooking 20 minutes in two cups of
water the same lot showed an average of 0.17% calcium or an
average loss of 46%.
Samples in Lot G showed an 8% loss of
calcium in the longer cooking period.
Therefore the average loss of calcium for the 15 minute period of cooking in one cup of water was 16%, while the
average loss for the 20 minute period in two cups of water
was 36%.
Cooking in a large amount of water, then, at
least doubled the loss of calcium in these experiments.
results might not be the same in cases where the kale was
The
18
not grown under similar conditions and cut at the same time
of the year.
Comparing these results with those of other investigators, Courtney, Fales, and Bartlett (14) reported that
carrots when boiled for 30 minutes showed a calcium loss of
28,A%..
In the same condition asparagus lost 26.6$, onions,
26..1% and Hew Zealand spinach 3.6$.
Peterson and Hoppert (34) analyzed raw beet greens
and found that they contained 0.0944 gm. of calcium per
100 grams of raw vegetable; raw cabbage and spinach gave
0..0338 gm. and 0.0763 gm. respectively..
They reported a
loss of 15.9% of the calcium when beet greens are boiled in
a large amount of water and a loss of 8.6$ when boiled in a
small amount of water.
In the case of cabbage they found a
loss of 72.2$ when cooked in a large amount of water and a
loss of 61.6$ when cooked in a small amount.
The salts of
cabbage seem to be particularly soluble in water and those
of spinach insoluble, as only a trace of calcium seems to
be lost when spinach is boiled for either a short or long
time.
Peterson and Hoppert report the average loss of calcium for all the vegetables to be 20 to 30$ when they are
boiled for thirty minutes.
The results of this investiga-
tion indicate that the losses of calcium in cooking kale by
these methods are within the range reported by Peterson
and Hoppert.
19
The calcium content of the raw kale was found to be
at least four times greater than that of any of the vegetables reported by Peterson and Hoppert, or by Courtney, Fales
and Bartlett.
In view of the fact that its calcium is well
utilized, kale therefore, becomes an excellent source of
calcium in the dietary.
SUMMARY AND CONCLUSIONS
20
The calcium content of kale was determined when raw
and xn/hen cooked in two ways:
boiled for 15 minutes In one
cup of water, and for 20 minutes in two cups of water.
An. average of 16% of the calcium of the kale was
lost when it was cooked for 15 minutes in one cup of water,
while an average of 36% of the calcium was lost when it was
cooked for 20 minutes in two cups of water.
Therefore, in this variety of kale, grown under these
conditions of soil and season, at least twice as much calcium was lost when the kale was cooked in a large amount of
water for a longer time than when cooked in a smaller amount
of water for a shorter time.
TABLE I
Calcium- Analyses of Raw Kale
1
Lot
Wt» of Kale
Gms.
Wt_ of Ca. in Sample
Gms.
Percent
Calcium
N.o..
Sample
Mo:.
A
I
57
.1795
0..31
D
I
57
.2295
0.40
G
I
57
.1607
0.28
11
57
.1607
0.28
.1826
0.31
Ave.
to
TABLE II
Calcium Analysis of Kale1 cooked 15 Minutes
In- a Small Amount of later
Lot
No..
Sample
Mo-
Wt.. of Kale
Gms.
Amount of
Water
Wt.. of Ca in Sample
Gms.
Percent Ca
In Sample
Percent
Ca Loss
&
ll
57
I cup
-1367
0.24
23
A
III
57
1 cup
.1367
0.24
23
A
K
57
1 cup.
.1512
0*2.6
15
A
V
57
1 cup
.1347
0.22
24
D
II
57
1 CUp;
.2142
0-36
10
D
III
57
1 cup
.2195
0.38
5
.1655
0.28
16
Ave.
to
to
TABLE III
Calcium- Content of Kale Cooked 20 Minutes
In a Large Amount of Water
Lot
Ho.
Sample
Ko.
It. of
Kale
Gm.,
Amount of
Water
Wt* of Ca
In Sample
Gma.
Percent
Ca
Percent Ca
Wt., of Ca in
Percent Cooking Liquid In cooking
Loss
Liquid
Gms.
A
VX
57
2 cups
.Q83Q
0..I4
54
.0417
0.07
A
VII
57
2 cups
.1087
0 J.9
39
.0765
0.13
D
m
57
2 cups
..IS 85
0.24
39
.0665
0.12
D
v
57
2. cups
►1390
0.24
39
.0540
0.08
G
in
57
2 cups
..1470
0.26
8
.0825
0.14
.1232
0.,21
36
.0642
0.11
Ave-
to
TABLE IV
SumiBary of Galciunr Analyses of Raw and Cooked Kale
Lot Wt. Sample
Mo.
Gms-
A
G
D
Kale co oked .15Mlir 1
Kale cooked 20 min. in 2 cups water
Raw Kale
In 1 c up water
I
Kale
Cooking Liquid
Wt ►Ca Percent Percent Wt.Ca
Wt.Ca. Percent Wt. Ca
Percent
Ca
Gnrs. % Ca. lLoss
Ca
Gms.
Gms
Loss
Gms
Ca
►1367
0..24
23
..0830
0.14
54
.0417
0.07
57
.1567
0.24
23
.1087
0*19
39
.0765
0.13
57
.1512.
0.26
15
57
.1347
0.22
24
.1470
0.26
8
.0825
0.14
57.
.1795
0.31
57
.1607
0^28
57
.1607
0.28
57
.2295
0.40
57
fivei age
.1826
0.51
.2142
0..56
10
.1390
0.24
39
►0665
0.12
»2195
0..38
5
.1385
0.24
39
.0540
0.08
►1655
0.28
16
.1232
0J31
36
.0642
0.11
I
to
BIBLIOGRAPHY
25
1.
Adolph, William H.. and Chen, Shen-Chao.. The utilization
of calcium in soy bean diets. J.
Nutrition,, 5:379-385, 1932.,
2.
Ascham, Leah..
3.
Bethke, E. I., Kick, C. H.. and Wilder,, Willard. The
effect of the calcium-phosphorus
relationship on growth, calcification
and blood composition by the rat.
J.. Biol., Chem., 98:389-403, 1932.
4..
Blatherwick, N. R.. and Long, Louisa M. Utilization of
the calcium and phosphorus of vegetables by man. J. Biol. Chem., 52:
125-131, 1922.
5.
Bloom, Margaret A.. The effect of crude fiber on calcium
and phosphorus retention. J.. Biol.
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